Fault protection circuit for an adjustable motorized hospital bed

ABSTRACT

The electrical control system for an adjustable motorized hospital bed is energized by an AC power supply through an isolation transformer. Power to the transformer&#39;&#39;s primary winding is interrupted in response to circuit component failure in the control system, which failure causes grounding of either terminal of the secondary winding to the conductive frame of the hospital bed. Each terminal of the secondary winding is coupled to the bed frame through a diode and a series-connected light emitting device. In the absence of component failure or fault, neither of the diodes conduct. However, when a failure or fault occurs and establishes a connection from one of the secondary terminals to the bed frame, the diode and light emitting device connected to the other secondary terminal conduct. The device illuminates and energizes a relay to open the primary winding circuit.

United States Patent Lawson 1 Oct. 29, 1974 [75] Inventor: Lance C. Lawson, St. Louis, Mo.

[73] Assignec: Borg-Warner Corporation, Chicago,

Ill

[22] Filed: May 4, 1973 [21] Appl. No.: 357,156

[52] US. Cl. 317/18 A, 317/124 [51] Int. Cl. 1102b 3/16 [58] Field ofSearch 317/18 R, 18 A, 18 C, 124; 324/51 [561 References Cited UNITED STATES PATENTS 3,168,682 2/1965 Moore et alt 317/18 A 3,441,801 4/1969 Kraus 317/18 R 3.515941 (1/1971) Moore etalm. 317/18A 3,659,152 4/1972 De Langis 317/18 8 Pu tlw o and Primary Examiner-James D, Trammell Attorney, Agent, or FirmJames E. Tracy [57] ABSTRACT The electrical control system for an adjustable motorized hospital bed is energized by an AC power supply through an isolation transformer. Power to the transformers primary winding is interrupted in response to circuit component failure in the control system, which failure causes grounding of either terminal of the secondary winding to the conductive frame of the hospital bed. Each terminal of the secondary winding is coupled to the bed frame through a diode and a seriesconnected light emitting device. In the absence of component failure or fault, neither of the diodes conduct. However, when a failure or fault occurs and establishes a connection from one of the secondary terminals to the bed frame, the diode and light emitting device connected to the other secondary terminal conduct. The device illuminates and energizes a relay to open the primary winding circuit.

1 Claim, 1 Drawing Figure Control System For Adjustable Motorized Hospital Bed #bed Frame PAIENIEflum 29 I914 mEPQ nmn m Gi nmwtopo 29595.25 Lou Empm m cO h. CJOLQ baasm Lm om U mZ FAULT PROTECTION CIRCUIT FOR AN ADJUSTABLE MOTORIZED HOSPITAL BED BACKGROUND OF THE INVENTION An adjustable motorized hospital bed, the positioning of which may be remotely controlled by the patient, must be completely safe to preclude any possibility of the patient being subject to an electrical shock. Many expedients have been employed to that end, including the grounding of the bed frame to earth ground and using transformers and photocells for electrically isolating the patient from the main AC power supply coupled to the bed.

It is also most helpful to effectively monitor the operation of the circuit components of the electrical control system for an adjustable hospital bed so that any component failure or fault, which causes the application of an undesirable voltage to the bed frame, will be immediately sensed and will result in turning off of all power to the bed. In this way, the troublesome component may be identified and repaired before the control system is made operable again.

The present invention provides a unique and inexpensive arrangement. for detecting a component fault or failure and for shutting down the entire system in response thereto.

SUMMARY OF THE INVENTION The fault protection circuit of the invention is to be incorporated in an adjustable motorized hospital bed wherein an AC power supply is coupled, via an isolation transformer, to a control system for the bed. A series circuit shunts the secondary winding of the isolation transformer and includes, in the order named, a first diode, a pair of light emitting devices and a second diode poled oppositely to the first diode. A relay controls a pair of normally closed contacts in the circuit coupling the power supply to the primary winding of the isolation transformer. Optically coupled to both of the light emitting devices is a photo-sensitive element. Means are provided for series-connecting the relay and the photo-sensitive element across the secondary winding. There are also means for grounding the junction of the light emitting devices to the conductive frame of the hospital bed. The relay energizes and opens the normally closed contacts, thereby disconnecting the primary winding from the power supply, anytime the failure of a circuit component of the control system results in connecting either terminal of the secondary winding to the bed frame.

DESCRIPTION OF THE DRAWING The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further advantages and features thereof, may best be understood, however, by reference to the following description in conjunction with the accompanying drawing which schematically illustrates a fault protection circuit, constructed in accordance with the invention, and the manner in which the circuit is incorporated in the electrical system of an adjustable motorized hospital bed.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Block represents a conventional AC power supply providing a single phase alternating voltage having a magnitude of approximately I I5 volts RMS and a commutating frequency of 60 cycles per second or hertz. This AC voltage, or what is commonly called line voltage, is usually available at a wall outlet in any hospital room where a hospital bed is located. The line voltage is produced across the output terminals labeled H and N in the drawing, those letters indicating the so-called Hot" and Neutral" terminals. The third output terminal of supply 10 is connected to earth ground.

Under normal conditions the line voltage is applied via a pair of normally closed relay contacts 12 to the primary winding 14 of an isolation transformer I5, which preferably has a l: l turns ratio. The terminals of secondary winding 16 connect over respective ones of line conductors L, and L to the electrical control system for an adjustable motorized hospital bed, depicted in the drawing by the single block labeled 19. In the absence of a fault or component failure, the v. line voltage is applied, via transformer I5, to control system 19 to effect energization thereof in order that the positioning of the hospital bed may be under control of the patient occupying the bed.

In atypical arrangement, the control system includes a pushbutton-actuated control device coupled to the motor-drive system of the bed. By selectively depressing the pushbuttons, the patient is able to remotely control the various adjustments. Usually, the mattresssupporting structure is articulated, being divided into four interconnected sections or panels, namely a back section, a center or seat section, an upper knee or thigh section and a lower knee or foot section. One motordriven adjustment that may be controlled by the patient raises or lowers the two knee sections where they join together, thereby controlling the position of the pa tients knees. Another adjustment, under the patient's control, pivots or tilts the back section with respect to the center section so that the patients back and head may be raised or lowered. In most cases, a third motordriven adjustment may be controlled by the pushbutton-actuated control device to vertically adjust the entire mattress-supporting frame.

As one example of a construction that control system 19 may take, reference is made to copending patent application Ser. No. l87,725, now abandoned, filed Oct. 8, 1971 in the name of Kenneth W. Padgitt. In that application, optically coupled circuits are employed to electrically isolate the patients hand control unit from the much higher current motor drive circuitry.

Fault protection is achieved by the series circuit comprising oppositely poled diodes 22 and 23, current limiting resistors 24 and 25 and light emitting devices 28 and 29, which may take the form of neon lamps. In addition, the protection circuit includes a relay 31 and a photo-sensitive element or photo resistor 32 optically coupled to both of the light emitting devices 28, 29. Preferably, relay 3] constitutes a mechanical reset latching relay which, in response to energization, opens contacts I2 and holds them open even after the relay deenergizes. A manual reset (not shown) would then be included to facilitate reclosing of contacts 12. Relay 3] and photocell 32 are series connected across secondary winding 16. The junction oflight emitting devices 28 and 29 is grounded to the bed frame, which of course is conductive, of the hospital bed. The grounded terminal of power supply 10 also connects to the bed frame.

During normal operation, namely when no fault exists, no current flows through lamps 28 and 29 due to the presence of diodes 22 and 23. Specifically, during the half cycles when line conductor L, is positive relative to conductor L diode 23 blocks any current flow through the series circuit, whereas during the alternate half cycles when line conductor L is positive with respect to conductor L diode 22 prevents the flow of current. With lamps 28 and 29 in their ofi or nonilluminated conditions, photo resistor 32 exhibits a resistance sufficiently high to prevent energization of relay 31. Contacts 12 thus remain closed.

Relay 31 energizes when a fault occurs in the control system such that one of its components fails and in so doing establishes an undesirable connection between the bed frame and either one of line conductors L and L Assume, for example, that conductor L becomes connected to the bed frame Diode 22, resistor 24 and lamp 28 become short circuited with the result that current flows through diode 23, resistor 25, and lamp 29 during the half cycles when line L, is positive relative to line L,. This current flow will be sufficient to illuminate lamp 29 and switch photo resistor 32 to its low resistance condition, causing relay 31 to energize and open contacts 12 to disconnect primary 14 from power supply 10. Power to control system 19 is thus interrupted and relay 3l deenergizes. However, contacts l2 have already latched into their open condition so that the entire system will remain inoperative until the troublesome component is located and repaired or replaced. After the trouble is corrected, the reset button is depressed to reconnect contacts 12 and reestablish the application of line voltage to primary winding 14.

Of course, if the fault occurred such that line conductor L were connected to the bed frame, current would flow through lamp 28 during the alternate cycles in which line L, is positive with respect to line L Lamp 28 would illuminate and cause photo-sensitive element 32 to exhibit its low resistance level, thereby effecting energization of relay 3].

The invention provides, therefore, a novel fault protection circuit for detecting when, due to circuit component failure, one side of the line voltage is present on the bed frame and for interrupting the power supply when such a condition is found.

While a particular embodiment of the invention has been shown and described, modifications may be made, and it is intended in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention.

1 claim:

1. A fault protection circuit for an adjustable motorized hospital bed wherein an AC power supply is coupled, via an isolation transformer, to a control system for the bed, comprising:

a series circuit shunting the secondary winding of the isolation transformer and including, in the order named, a first diode, a first current limiting resistor, a pair of light emitting neon lamps, a second current limiting resistor, and a second diode poled oppositely to said first diode;

a relay for controlling a pair of normally closed contacts in the circuit coupling the power supply to the primary winding of the isolation transformer;

a photo-resistor optically coupled to both of said neon lamps and having a relatively high resistance when both of said neon lamps are off and a relatively low resistance when either one of said neon lamps is on;

means for series-connecting said relay and said photo-resistor across the secondary winding;

and means for grounding the junction of said neon lamps to the conductive frame of the hospital bed,

said relay energizing and opening said normally closed contacts, thereby disconnecting the primary winding from the power supply, any time the failure of a circuit component of the control system results in connecting either terminal of the secondary winding to the bed frame,

t i l l 

1. A fault protection circuit for an adjustable motorized hospital bed wherein an AC power supply is coupled, via an isolation transformer, to a control system for the bed, comprising: a series circuit shunting the secondary winding of the isolation transformer and including, in the order named, a first diode, a first current limiting resistor, a pair of light emitting neon lamps, a second current limiting resistor, and a second diode poled oppositely to said first diode; a relay for controlling a pair of normally closed contacts in the circuit coupling the power supply to the primary winding of the isolation transformer; a photo-resistor optically coupled to both of said neon lamps and having a relatively high resistance when both of said neon lamps are off and a relatively low resistance when either one of said neon lamps is on; means for series-connecting said relay and said photo-resistor across the secondary winding; and means for grounding the junction of said neon lamps to the conductive frame of the hospital bed, said relay energizing and opening said normally closed contacts, thereby disconnecting the primary winding from the power supply, any time the failure oF a circuit component of the control system results in connecting either terminal of the secondary winding to the bed frame. 